The 20-year-old Sumatran orangutan had been contending with an illness for about a month and responding well to treatment, according to zoo officials. But Kutai’s condition deteriorated Saturday morning and he died following an afternoon surgery.

Kutai, who has been in Portland since 2001, was described as “extremely intelligent and creative” in an Oregon Zoo press release.

Kutai was born at Sedgwick County Zoo in Kansas. Kutai’s 54-year-old grandmother, Inji, is the oldest animal at the Oregon Zoo.

Zoo staff performed a necropsy Saturday night, which may give a better sense of why the orangutan’s condition deteriorated so rapidly.